Capped antenna of the offset cassegrainian type

ABSTRACT

In this antenna, the auxiliary reflector and the aperture of the primary source are located above the main reflector. A cap having a metal layer and an inner absorbent layer covers and surrounds the aperture of the primary source and the whole of the auxiliary reflector. In addition to its radioelectric function, the cap thus also plays the part of a radome.

Unite States Patent 1 1 1 3,810,187 Hai 5] May 7, 1974 [54] gggggg igffiggag OFFSET FOREIGN PATENTS OR APPLICATIONS 1,128,952 9/1956 France 343/781 [75] Inventor: Nhu Bui Hai, Paris, France [73] Assignee: Thomson-CSF, Paris, France Primary Examiner-Eli Lieberman Attorney, Agent, or FirmCushman, Darby & [22] Filed. Apr. 18, 1973 Cushman 1211 Appl. No.: 352,332

[57] ABSTRACT [30] Foreign Application Priority Data Apr. 21, 1972 France 72.14251 this antenna, the auxiliary reflect and the P ture of the primary source are located above the main 52 us. Cl 343/781, 343/837, 343/872 reflector- A Cap having a metal layer and an inner 511 1m. (:1. nolq 19/14 Sorbet layer covers and Surrounds the aperture of the [58] Field of Search 343/781, 837, 840, 872 Primary Source and the Whole of the auxiliary reflector. In addition to its radioelectric function, the cap [56] References Cited thus also plays the part of a radome.

UNITED STATES PATENTS 4 Claims, 3 Drawing Figures 3,733,609 5/1973 Bartlett 343/840 CAPPED ANTENNA OF THE OFFSET CASSEGRAINIAN TYPE The present invention relates to a very wide-band high-efficiency microwave antenna of the offset Cassegrainian type.

It is well-known to associate with a radiating aperture, this for the purpose of reducing the diffraction due to its edges and the parasitic reflections produced by neighbouring metal structures, and thus of reducing the amplitude of the diffuse lobes, a screen lined with an absorbent material.

In offset Cassegrainian antennas which comprise a primary source, an auxiliary reflector and a main reflector forming the last element of a transmitting an tenna (and the first element ofa receiving antenna) the radiating system formed by the aperture of the primary source and the auxiliary reflector is located above the main reflector. It has been proposed, in a German patent application (Offenlegungsschrift 1,516,828) to associate with such an antenna a screen having the shape of a dish laterally surrounding this radiating system.

The purpose of the present invention is an offset Cassegrainian antenna in whichthe conventional position of the elements is so modified and the fixing system so arranged, that the screen, in addition to its double radio-electrical function (absorption of the diffracted waves and reflection of the external parasitic waves) affords a climatic protection, this without mechanical drawbacks. The use of a radome surrounding the antenna and the resulting loss of energy may thus be avoided.

In accordance with the invention, there is provided an offset Cassegrainian antenna comprising a chassis, a primary source having an aperture, an auxiliary reflector, a main reflector, a screen lined with a microwave absorber, and means respectively securing said primary source, said auxiliary reflector, said main reflector and said screen to said chassis; said aperture of said source and said auxiliary reflector being located above said main reflector; said screen having the form of a cap covering said auxiliary reflector and said primary source; and said means securing said screen to said chassis being independent from said means securing said auxiliary reflector to said chassis.

Other features will become apparent and the invention will be better understood, from a consideration of the ensuing description and the related drawings in which:

FIG. 1 is a schematic view of an antenna in accordance with the invention;

FIG. 2 is a section through one of the elements of FIG. 1;

FIG. 3 is a more detailed view ofa part of the antenna of FIG. I.

In the different drawings, the corresponding elements are marked by the same symbols.

FIG. 1 illustrates a primary source, 1, of the horn type with a reflector. In order not to overburden the drawing, the elements terminating in this primary source and designed to feed power to it for radiation, have not been shown. An auxiliary reflector 2 and a main reflector 3, complete the radiating part of this offset Cassegrainian antenna. The primary source 1 and the main reflector 3 are attached to a tubular chassis 4 by means of nut-bolt devices not shown.

The elements of the chassis 4 are welded together. They comprise three supports, a front one, 40, and two, 41 (only one of which can been seen), at the rear, these three supports being located at the apices of an isosceles triangle, the sides of which are formed by three horizontal tubes such as tube 45. Two transverse tubes 42 (only one of which can be seen) leaning backward, have their first ends fixed to the support 40, while two other transverse tubes 43 (only one of which can be seen) leaning forward. have their first ends respectively secured to the two supports 41. The two transverse tubes 42 have their second ends respectively fixed to the tubes 43, thus forming a V. Together with the two tubes 42, two small tubes 44 respectively secured to the two tubes 43, are used for supporting the main reflector 3.

A tubular frame 5 has a rear end secured to the tubes 43, and a front end supporting the auxiliary reflector 2.

The antenna is fitted with an element 6 which will be referred to as a cap. This cap is attached to the tubular chassis 4. The auxiliary reflector 2 and the aperture of the primary source 1, which faces the auxiliary reflector, are hidden by this cap which also covers the tubular frame 5; these hidden parts of the antenna have been illustrated in broken lines in FIG. 1. The cap 6 is not directly fixed either to the primary source 1, or to the auxiliary reflector 2, or to the tubular frame 5, and can therefore shift under the effect of the wind without involving in its motion the tubular frame and the auxiliary reflector.

The cap has a curved surface, having two symmetry planes. Its section in the longitudinal symmetry plane has approximately the form of a half-ellipse subtended by its major axis, as shown in FIG. 1, and its section in the transverse symmetry plane approximately that of a semi-circle, as shown in FIG. 2 which is a section view of the cap through this latter plane, marked X-X in FIG. 1.

This drawing shows the three superimposed layers of which the cap is made, and these are, commencing from the concave face:

a microwave absorber 8, made up of graphite-filled urethane foam, the outer side of this absorber being formed by juxtaposed small cones;

a metal layer constitted by a copper weave with close meshes, 9;

a stratified polyester layer, 10, used as a support for the metal layer, 9.

In order not to overburden the drawings, the fixing system of the cap has not been shown either in FIG. 1 or in FIG. 2.

This fixing system is shown in FIG. 3. It comprises four curved tubular elements 11, 12, 13 and 14 the greatest part of which is located inside the wall of the cap, beneath the absorber layer. Those four elements are joined together, at one end, at the front part 15 of the cap, the other ends of the elements 11 and 12, which follow the edge of the cap, being respectively fixed to the two tubes 43 of the chassis 4. The elements 13 and 14 form two arcs respectively located on either side of the longitudinal symmetry plane of the cap, and their second ends are also secured to the tubes 43.

The function of the cap is at the same time radioelectrical, mechanical and climatic. From the radioelectrical point of view, the cap, on the one hand, clue to its metal layer, forms a screen as concerns external parasitic radiations, and on the other hand, due to the absorber layer, intercepts virtually the whole of the energy radiated by the primary source 1 and not captured by the auxiliary reflector 2, and part of the energy diffracted by the edges of the primary source 1, by the edges of the auxiliary reflector 2 and by the tubular frame 5. The energy thus intercepted is either absorbed directly by the absorber 8, FIG. 2, or absorbed after reflection by the copper weave 9, FIG. 2, in the case of energy which has passed through the absorber; thus the cap makes it possible to reduce to 0.2 dB the undulation in the gain-frequency curve of the antenna, whereas it was around 1 dB in the operating frequency band of 3.8 to 7.l Gc/s in the antenna taken by example; it likewise makes it possible to reduce by dB the level of the radiated diffuse lobes, thus increasing the number of antennas of the described kind which it is possible to install at a junction of main telephone routes and, correspondingly, to increase the number of directions in which transmission can be made.

The mechanical function of the cap stems from the fact that due to its position, it protects against the effects of wind part of the primary source, the tubular assembly and the reflector; thus, the stresses produced by windloads on the auxiliary reflector being very much reduced, the diameters of the tubes used in the tubular assembly can be reduced correspondingly, bringing about a reduction in the parasitic diffraction effects for which these tubes are responsible.

From the climatic point of view, the cap protects the auxiliary reflector against direct rain and snow, thus improving the reliability of operation of the antenna.

Antennas such as that just described, are intended more especially for radio links, but may also be used in the space and radar fields.

Self-evidently, the invention is not limited to the embodiment described and it is possible, bu suitable arrangements, to continue the cap so that it covers that part of the main reflector 3 shown in FIG. 1, which is opposite the auxiliary reflector 2.

What is claimed is:

1. An offset Cassegrainian antenna comprising a chassis, a primary source having an aperture, an auxiliary reflector, a main reflector, a screen lined with a microwave absorber, and means respectively securing said primary source, said auxiliary reflector, said main reflector and said screen to said chassis; said aperture of said source and said auxiliary reflector being located above said main reflector; said screen having the form of a cap covering said auxiliary reflector and said primary source; and said means securing said screen to said chassis being independent from said means securing said auxiliary reflector to said chassis.

2. An offset Cassegrainian antenna as claimed in claim 1, wherein said screen comprises an outer supporting layer and between said supporting layer and said microwave absorber, a reflecting metal layer.

3. An offset Cassegrainian antenna as claimed in claim 1, wherein the outer side of said microwave ansorber is formed by juxtaposed small cones.

4. An offset Cassegrainian antenna as claimed in claim 1, wherein said cap has a curved surface having two symmetry planes, the sections of said surface through said symmetry planes being close to a halfellipse subtended by its major axis and to a half-circle respectively. 

1. An offset Cassegrainian antenna comprising a chassis, a primary source having an aperture, an auxiliary reflector, a main reflector, a screen lined with a microwave absorber, and means respectively securing said primary source, said auxiliary reflector, said main reflector and said screen to said chassis; said aperture of said source and said auxiliary reflector being located above said main reflector; said screen having the form of a cap covering said auxiliary reflector and said primary source; and said means securing said screen to said chassis being independent from said means securing said auxiliary reflector to said chassis.
 2. An offset Cassegrainian antenna as claimed in claim 1, wherein said screen comprises an outer supporting layer and between said supporting layer and said microwave absorber, a reflecting metal layer.
 3. An offset Cassegrainian antenna as claimed in claim 1, wherein the outer side of said microwave ansorber is formed by juxtaposed small cones.
 4. An offset Cassegrainian antenna as claimed in claim 1, wherein said cap has a curved surface having two symmetry planes, the sections of said surface through said symmetry planes being close to a half-ellipse subtended by its major axis and to a half-circle respectively. 